1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and more particularly, to a liquid crystal display apparatus having a wide viewing angle.
2. Description of the Related Art
As the screen size and capacity of the liquid crystal display apparatuses increases, vigorous efforts have been made to increase the use of liquid crystal display apparatuses, which utilize the electrooptic characteristics of a liquid crystal, in wider applications in the field of office automation equipment. Liquid crystal display apparatuses currently in common use are classified by their respective operation modes. Generally, there are two types; twisted nematic (TN) type wherein liquid crystal molecules interposed between two glass substrates show an orientation twisted 90.degree., and super twisted nematic (STN) type wherein the liquid crystal molecules show orientations twisted 180.degree. to 270.degree.. The TN type is used mainly in active matrix liquid crystal display apparatuses, and the STN type is used in simple matrix type liquid crystal display apparatuses. These apparatuses will be described below with reference to the accompanying drawings.
In the case of the TN type used in active matrix type liquid crystal display apparatuses or in liquid crystal display apparatuses of small sizes, the liquid crystal molecules in the interface with the glass substrate are uniformly oriented in the same direction at a pre-tilt angle to the glass substrate, with the orientation being twisted 90.degree. between the upper and lower glass substrates. The 90.degree. twist in the orientation is obtained, in general, by rubbing rayon fabric or the like against an alignment film comprising a thin polyimide film formed on the glass substrate in one direction, and arranging the upper and lower substrates such that the directions on the two substrates are at right angles to each other.
When a voltage is applied to a TN type liquid crystal display apparatus as shown in FIG. 1, the liquid crystal molecules 101 in the state of orientation twisted 90.degree. begin to respond. As the voltage increases beyond the threshold voltage, the state of twisted orientation decays and replaced by splay orientation, so that the major axes of the liquid crystal molecules 101 are caused to erect out of the plane of the glass substrates 102 and 103. When the liquid crystal molecules 101 are observed in a direction of angle from the normal line (Z axis) of the substrate while changing the azimuth .PHI., the major axes of the liquid crystal molecules 101 are not uniform in the direction of azimuth angle. As a result, apparent anisotropy of the index of refraction ( .DELTA.n) of the liquid crystal molecules 101 changes with the azimuth angle, thereby changing the amount of birefringence (.DELTA.nd) which is the apparent anisotropy of the index of refraction multiplied by the thickness of the liquid crystal layer (d). Therefore, when polarizers are arranged on the outward surfaces of the upper and lower glass substrates 102 and 103 so that light absorbing axes 106 and 107 lie perpendicular to the rubbing directions 104, 105 and light is allowed to enter in the -Z axis direction, the intensity of transmitted light varies with the azimuth angle thereby causing asymmetry with respect to the viewing angle. This asymmetry in the viewing angle poses problems. Particularly during display of half tones or gray scales, problems such as significant loss of contrast depending on the angle of view or deterioration in the picture quality such as reversal of the displayed image may occur. For this reason, a proposal to expand the viewing angle of a TN type liquid crystal display apparatus has been made by Toko, Kobayashi, et al. (Y. Toko, T. Sugiyama, K. Katoh, Y. Iimura, S. Kobayashi: SID 93 DIGEST, pp. 622-625 (1993)). According to the proposal, a liquid crystal cell is formed by injecting a chiral nematic liquid crystal into the cell while keeping a specified spacing without rubbing after forming the polyimide alignment film on a pair of electrodes. Switching of light by the application of voltage under parallel Nicol prisms or perpendicular Nicol prisms is made possible by setting the chiral pitch of the chiral nematic liquid crystal to four times the cell thickness.
However, the liquid crystal display cell fabricated without rubbing proposed by Toko et al. requires the injection into the cell of a liquid crystal in the isotropic phase, which leads to such drawbacks as the necessity of new facilities to heat the entire liquid crystal cell above the isotropic phase temperature during injection, thereby making the proposal impractical. In the case where a liquid crystal cell without rubbing is fabricated by employing the method of injecting the liquid crystal in nematic phase, which has been employed in the manufacture of TN liquid crystal display elements, the liquid crystal is only partially oriented because of the liquid crystal flow during injection resulting in a drawback of lack in homogeneity which can be recognized by unaided eyes. Further, in such liquid crystal cells which have partially oriented due to the liquid crystal cell during injection, the inhomogeneity cannot be completely eliminated even after keeping the cell in the isotropic phase for several hours.
Also in the case where a liquid crystal display cell is fabricated without rubbing, it is difficult to control the texture of the liquid crystal which is formed by cooling. Therefore, when observed in an oblique direction under crossed Nicol prisms, unevenness of the luminance sometimes becomes so large so as to cover a plurality of pixels, causing the problem of appearing rough to the eye.